I've been through the whole process of big guns. 459 mags, weatherby calibers, the 50 alaskan. I've owned and hunted with them all. Shotguns from 410 through 10ga. And up to the 454 casule in the handguns. In bows I used 85-90 lb recurves & longbows. Believe me the human body will not stand up to that kind of punishment forever. It will eventually break you down.
I'll be 65 in a couple of months and I understand where your comming from, I didn't believe it would at your age either, BUT IT WILL.
The 3&1/2" 12 gague is brutal, if I'm going to shoot a 3&1/2" shell I would reater it be a 10ga. However I still maintain it is not needed for a 20lb bird.
I've killed turkey & geese with 20,16,&12 ga guns shooting high brass 2&3/4" shells just fine if you will simply get within range.
If your a skybuster you'll miss a lot regardless of the load....JME
Think about this, why are all our middle age athlets so broken down? The hunan body will only stand so much. Take care of yours you only get one.....
regards,